Figure 5.7: MESA dynamic spectra for magnetic field components as measured by the CHAMP satellite on 15/02/2003. Also shown are the geocentric latitude and L-shell values during the satellite traverse.
900 rotation of the magnetic field components due to ionospheric currents is clearly observable.
Figure 5.8: CHAMP (top panel) and ground (bottom panel) wave hodograms for three consec- utive 20 second intervals at the time when the satellite was passing over the Hermanus ground station.
However, this discussion centres around the time when the satellite was passing over the HER ground station.
The MESA dynamic spectra for the CHAMP data show multiple frequency structures that vary with time. These variations resemble the intermittent nature and variations in period observed by Ver˜o et al. (1998) in UW pulsations at ground stations. The dynamic spectra show that the Bcom and Bpol components tend to oscillate at similar frequencies. For example they both exhibit oscillations at 45 and 65 mHz during the first three minutes of the Pc3 event;
however, the Bpol component decreased to 40 mHz at the time that the satellite approached the HER latitude and then decreased further to 35 mHz. During the last three minutes there are clear oscillations at 25 and 45 mHz and evidence of oscillations at 75 mHz. Consequently, our results are not in full agreement with Vellante et al. (2004), who reported the Bpol frequency to be 20% higher than Bcom and which they ascribed to a Doppler effect as a result of the rapid motion of CHAMP across field lines. Our observations for the FLR driven by Bcom at 65 mHz indicate that Bpol did not experience a Doppler shift. However, for the FLR driven by Bcom at 45 mHz, the frequency of the Bpol component decreased at the time that CHAMP crossed over HER. In the non-uniform plasma of the real magnetosphere, the fast and transverse waves have a degree of coupling, and in such case then Bpol might be expected to be affected by a FLR as shown by Hughes and Southwood (1976). In their model it was demonstrated that phase variation and consequently the Doppler shift of Bpol around the resonance is less drastic but
can not be completely ignored. This can explain the frequency decrease of Bpol when CHAMP approached HER latitude. Despite the intermittent nature of the structure in the spectra and the changing latitude of CHAMP, some frequencies seem to persist, e.g. 25 mHz and 45 mHz.
This is reminiscent of the results of Menk et al. (2000), who found power spectra for a ground magnetometer array to be similar over a range of latitudes.
The dynamic spectra of Bcom, which acts as the source wave, and Btor, which represents the driven or forced wave, exhibit some correlation. However, the nature of this correlation is dif- ferent between the parts of the spectrum where Btor intensities are high and the parts where intensities are low. The short intervals of intense oscillations, centred at 11:17.5 UT and 11:19 UT, are interpreted as times when CHAMP is crossing a field-line resonance. The observation of an apparent Doppler shift in Btor frequency for the first part of the dynamic spectrum where intensities are high agrees with Vellante et al. (2004), who reported the Btor frequency to be 20% higher than Bcom. However, in our case the Doppler shift is to lower frequencies due to the satellite moving poleward rather than equatorward. In contrast with the first part of the spectrum, the last three minutes of the Btor spectrum shows clear oscillations of much lower intensity at three frequencies as in Bcom and Bpol. The latter are not subject to a Doppler shift. The absence of any Doppler shift at these frequencies can be explained by the oscillations not being associated with a FLR. The Btor oscillations arise due to weak coupling between the fast mode and Alfv´en mode waves. Note that these three frequencies are observed when CHAMP traverses magnetic shells with L≥2.4 and at these L-values the resonant frequency is expected to be <20 mHz according to previous observations (e.g. Menk et al., 2004 and Dent et al., 2006). Consequently, the absence of a FLR and the associated Doppler shift at these frequencies is justifiable.
At low latitudes, we expect plasma density, magnetic field intensity, and field line length to vary smoothly with latitude. Consequently, the field line resonant frequency is also expected to vary smoothly and continuously as a function of latitude. However, resonant field line oscillations will only be excited at latitudes where there is a matching driving force, i.e. a fast mode wave.
Bcom, which is the signature of a fast mode wave, exhibits oscillations at frequencies 65 and 45 mHz from 11:16 to 11:20 UT. Consequently, we can expect toroidal mode oscillations to occur at these frequencies at latitudes where they match the field line resonant frequency. In Figure 5.7 we see that for a driving frequency of 65 mHz in Bcom, this occurs at latitudes centered on 270 S as evidenced by the short interval of intense oscillations in Btor. Note, however, that the Btor frequency observed on CHAMP is Doppler shifted to a lower frequency of 60 mHz.
Vellante et al. (2004) provide an explanation and derivation of this Doppler shift, which is due to the rapid rate at which a LEO satellite traverses the rapid phase change across the field line
resonance region. In the case of the example given by Vellante et al. (2004) the Doppler shift was to higher frequencies due to CHAMP moving equatorward. In our example the Doppler shift is to lower frequencies due to CHAMP moving poleward. Ninety seconds later CHAMP crossed another resonance centered at 340 S, where the Bcom driving frequency is 45 mHz and the Btor resonance frequency is shifted to 40 mHz as observed on CHAMP. The latter latitude is that of HER and the fast mode frequency matches the resonant frequency observed in the H-component on the ground at HER as shown in Figure 5.6.
In comparing the satellite and ground dynamic spectra we note that the compressional wave in the ionosphere has similarities with the D-component on the ground. Similar trends were also observed by Vellante et al. (2004) and confirm Odera’s observations (Odera et al., 1991), who suggested that Pc3 energy is transported through compressional fluctuations. The hodograms computed confirm the 900 rotation of the magnetic field components.